The present invention relates to a signal transmission system in a communication network system wherein one central equipment unit and a plurality of terminal equipment units are connected via a signal transmission line and, more particularly, to a system for adjusting a signal transmission timing at each terminal equipment unit, for the purpose of preventing signal collisions on the signal transmission line.
A communication network is known, which is constructed such that a terminal equipment unit having concentration and distribution functions is installed in a building or at a floor of the building, at least one terminal device such as a telephone set is connected to the terminal equipment unit, and a plurality of such terminal equipment units are coupled to a central equipment unit such as a PBX (private branch exchange) via an up link and a down link.
In such a network, the terminal equipment units are connected at different positions along the signal transmission line. For this reason, the signal propagation times (transmission delay times) between the respective terminal equipment units and the central equipment unit differ from each other. In order to transmit signals on a time-division multiplexing basis from a plurality of terminal equipment units to the central unit via the common transmission line, it is very important to prevent collisions, on the signal transmission line, of signals transmitted from the terminal equipment units. In order to prevent the signal collisions, it is necessary to adjust the signal transmission timings of the terminal equipment units, taking the different transmission delay times thereof into consideration. However, simply setting time margins for signal transmissions while taking into consideration the different transmission delay times of the terminal equipment units would considerably reduce the utility efficiency of the transmission line.
U.S. Pat. No. 4,594,705, issued June 10, 1986; entitled "BUS-CONFIGURED LOCAL AREA NETWORK WITH DATA EXCHANGE CAPABILITY"; and assigned to the same assignee as this application, discloses a system or adjusting a signal transmission timing of a terminal equipment unit. This system operates in a test mode, a transmission-start timing setting mode, and a normal data transmission mode. In the test mode, a central equipment unit transmits test signals addressed to the terminal equipment units, and each terminal equipment unit transmits a response signal to the central equipment unit, in response to the reception of the test signal addressed to itself. The central equipment unit measures a delay time between the transmission timing of the test signal and the reception timing of the response signal transmitted from the terminal equipment unit. In the transmission-start timing setting mode, the central equipment unit transmits control data based on the measured delay time to each terminal equipment unit. In the transmission mode, each terminal equipment unit transmits a data signal to the central equipment unit at a timing based on the control data transmitted from the central equipment unit.
According to the system, since the delay time measurement mode and the data transmission mode are provided independently from each other, if an additional terminal equipment unit is connected to the network, it is difficult to measure a transmission delay time between the additional terminal equipment unit and the central equipment unit. In other words, the delay time between the additional terminal equipment unit and the central equipment unit must be measured when the existing terminal equipment units are not busy.
U.S. Pat. No. 4,694,453 issued Sept. 15, 1987, entitled "SYSTEM FOR ADJUSTING SIGNAL TRANSMISSION TIMING IN TIME-DIVISION MULTIPLEXING SIGNAL TRANSMISSION", and assigned to the same assignee as this application, discloses a time-division multiplexing communication network in which each terminal equipment unit measures a transmission delay time between the central equipment unit and itself, so as to adjust the timing for a signal transmission to the central equipment unit.
According to the transmission system disclosed in this patent, the central equipment unit requests each terminal equipment unit to transmit a test signal, and the terminal equipment unit requested to do so, transmits the test signal. The central equipment unit sends the test signal, transmitted from the terminal equipment unit, back to the terminal equipment unit, without performing any processing. The terminal equipment unit measures a transmission delay time between the transmission timing of the test signal and reception timing of the test signal sent back from the central equipment unit, and adjusts the transmission timing of a data signal for the central equipment unit in accordance with the measured transmission delay time.
One transmission frame of the system includes a data transmission interval having time slots assigned to the respective terminal equipment units, and a window interval for measuring delay times. The window interval includes an address field for designating one of the terminal equipment units to transmit the test signal, and a test signal transmission/receiving field in which a terminal equipment unit transmits a test signal to the central equipment unit and receives the test signal sent back therefrom. Only the test signal transmission/reception field is used to transmit/receive the test signal for delay measurement, and the address field cannot be used therefor. For this reason, a time interval of a delay measurement window must be set relatively long, thus reducing the utility efficiency of time slots for data transmission. In addition, since each terminal equipment unit must receive a data signal transmitted from the central equipment unit and the test signal transmitted by itself, it is difficult to stably receive these signals. In other words, an expensive receiving device is required to ensure stable reception of the signals.